When doing partial blob update with NdbBlob::writeData(), zero-padding
after the write was wrongly done, causing part of the old blob value
to be overwritten with zeros (or spaces for text field).

Fixed by only padding when needed (when writing at end of the blob).


ndb/src/ndbapi/NdbBlob.cpp:
  Do not pad rest of blob part after the write, unless it is a write at the
  end of the blob.
ndb/test/ndbapi/testBlobs.cpp:
  Add test case.